Temperature compensation means for fixed reactances in tunable circuits



' April 20, 1948. T. A. HUNTER 2,439,809 TEMPERATURE COMPENSTIONMEANSFOR FIXED REACTANCES IN A TUNABLE CIRCUIT A Filed Feb. l, 1943 2Sheets-Sheet 2 @W95/750 MQW 60H5 OFWE/I/ATER//m J? mmm m M/w/ f I Y 7 mwm: fw/Ma w ,w y y..\ \M, W 4p Y wf .5, @n www 5. W, I m I w,

Patented Apr. 20, 1948 TEMPERATURE COMPENSATION MEANS FOR FIXED REACTANCES IN TUNABLE CIRCUITS Theodore A- Hunter, Iowa City, Iowa, assignor toCollins Radio Company, a corporation of Iowa Application February 1,1943, Serial No. 474,371

(Cl. Z50-40) 12 Claims. 1

This invention relates to temperature compensation, and moreparticularly to adjustable means for compensating for variations in theimpedance of the tuned circuit and elements thereof as a result oftemperature variations.

One feature of this invention is that it provides means 4for accuratelycompensating for variations which would otherwise be occasioned bytemperature variations; still another feature of this invention is thatit provides means for readily adjusting the compensation factor orcoeflicient to meet manufacturing variations and to permit accuratecompensation under production conditions; yet another feature is thatsuch adjustment may be made without changing, at

the temperature at which the adjustment is being made, the impedance ofthe circuit element with which the compensating means is associated; anda further feature is that the temperature compensating means is ruggedand stable under adverse conditions, as heavy vibration. Other featuresand advantages of this invention will be apparent from the followingspeciiication and drawings, in which:

Figure 1 is a side elevation, partly broken away, of an oscillatortuning unit embodying a preferred form of my invention; Figure 2 is afront view of the unit shown in Figure 1; Figure 3 is a fragmentarytransverse sectional view along the line 3-3 of Figure 1; Figure 4 is a'schematic circuit representation of this tuning unit; Figure 5 is aview, principally in longitudinal section, of another embodiment of myinvention; Figure 6 is a front view of a tuning unit embodying stillanother modification of my invention; Figure 'I is a side view, partlybroken away, of this unit; Figure 8 is a sectional view, partly brokenaway, along the line 8-8 of Figure 6; and Figure 9 is a transversedetail view, principally in section, along the line 9-9 of Figure '7. Inso far as they are known to applicant, previous attempts to providecompensation for variations in impedance of a circuit element uponvariations in temperature have been in connection with the condenser ofa tuning circuit, generally by the provision of a separate smallcondenser whose sole function was temperature compensation. Such anarrangement, however, provides the same amount of capacity change perunit of temperature change regardless of the frequency to which thecircuit may be tuned, rather than vproviding a compensation which wasalways a given percentage of the condenser reactance, as it should be;and adjustability of the compensation coefficient (the amount of changeof reactance per unit of temperature change) was lacking. My inventionobviates these disadvantages, provides a very stable and ruggedcompensating arrangement with a linear action and with means for readilyadjusting the compensation coeillcient, this latter being very importantin that it enables manufacturing variations to be compensated for inproduction.

In the particular embodiment of my invention illustrated in Figures 1-4a tuning unit, adapted to be associated with an oscillator, is indicatedin general as I0. This unit comprises a coil Ii in parallel with avariable condenser I2 to pro- Vide the tank circuit of an oscillatorwhen properly associated with a tube and other elements. Even thougheveryelort is made to provide the minimum temperature coemcient of thesecircuit elements, the tank circuit will usually have a positivetemperature coeilicient of 10 or 15 parts per million; that is, itsimpedance increases to this extent per degree of temperature rise, thefrequency to which it is resonant similarly decreasing. Where no specialeffort is made to eliminate the effect of temperature variations a tankcircuit will commonly have a positive coefficient of from 30 to 100parts per million per degree centigrade.

Where a tuning circuit must operate through Wide ranges of temperature,as in an aircraft radio, this temperature change can cause a veryappreciable frequency change. It is to prevent this change that I haveintroduced temperature compensation.k Previous efforts in this regardhave been unsatisfactory for a number of reasons, the principal onebeing that they lacked ready adjustability. In commercial production twosimilar tuned circuits may have widely different temperature coemcients,and accurate and satisfactory `compensation cannot be obtained except byan arrangement which permits each tuning circuit compensating means tobe acljusted to meet its particular conditions.

I have found that it is much more advantageous to provide temperaturecompensation in connection with the coll of a tuned circuit rather thanin rconnection with the condenser. It is now becoming quite common tointroduce a, permeability core (comprising fine metallic particles in anappropriate binder or base material) into a coil to increase itsinductance for a given size and number of turns and I take advantage ofthis by providing for movement between the core and coil to effect thedesired temperature compensation, this movement being automatiallyelected by temperature responsive means.

Depending upon the direction and extent of movement provided in thisway, any desired temperature coefiicient, either positive or negative,may be provided for the compensating action.

In the embodiment now being discussed a permeability core I3. is mountedon a. shaft I4 of insulating material, this shaft being threaded into anut I5 carried by a bi-metal strip or plate l5. The bi-metal plate hasits four corners rigidly connected, as by bolts II and spacers I8 to thefront plate I9 of the tank. circuithousing. This plate is provided withslots` 20 and 2|, and the bi-metal plate I6 is provided with registeringslots 22 and 23, these slots being symmetrically arranged on each sideof the shaft I4V with their axes lying along a line through the centerof such shaft. An adjustment bolt 24 isadaptedV to pass through and bemovable in the slots 20 and 22, this bolt being provided with acooperating spacer 25; andv a similar adjustment bolt 26 is movable inthe slots 2| and- 23 and is provided with a spacer 27.

As may be best seen in Figure 3, the bi-metal plate i5 is here arrangedto bulge outwardhl upon increase in temperature, the core being` movedout of the coil by this movement ofthe center of the plate to reduce itseffect upon the coil (and thus the inductance ofthe coil) as thetemperatures rises. At low temperatures the strip would straighten outmore, as shown in dotted lines, resulting in an increased effect of thecore upon the coil inductance.

As long as the movement of the core is` such that the inductance of thecoil does not too closely approach that at the ends. of the permeabilitytuning curve the change. of inductance per unit of movement will belinear for all practical purposes; and a bi-metal. strip or plate of.the character shown has a lineal movement, in response to temperaturechanges, within three or four per cent, throughout ranges from 100 aboveZero to 50 (centigrade) below, the action still remaining reasonablylinear even beyond such temperatures. It will thus be, apparent that avery accurate and linear temperature compensation action is accomplishedthroughout wide temperature ranges.

The amount of inductance change per unit of shaft movement may beinitially determined by the size of the core used in relation to a givencoil, and by ie spacingbetween the turns of the coil, so that such aunit can be designed to provide a positive coeiiicient in` theneighborhood of parts per million or in the neighborhood of 100 partsper million, or practically any compensation that may be desired',either positive or negative. Accurate adjustment of the compensationcoefficient to the conditions of the particular elementA or circuit maythereafter be made by movement of the adjusting bolts 24 and 26. Inwhatever position these bolts are placed they immobilize the ends orportions of the bimetal strip lying beyond them, and the positioning ofthese bolts therefore. determines the effective length of theactivecenter. portionroi the strip, and thus the movement of the shaftper degree change of temperature. In the particular embodiment of myinventionillustrated herein, with elements designedy to have lowtemperature coefficients, movement of the adjusting bolts from theirposition closest to the shaft to their position farthest removedtherefrom resulted in a variation of the compensation action between 6and 1S parts per million per degree vcentigradffl a sufficient range toenable accurate compensation for manufacturing variations and veryprecise maintenance of the desired frequency of the tuned circuit duringwide ranges in the temperatures at which it operated.

In making the final adjustments on the oscillator the workman firsteffects the desired factory adjustment to a desired operating frequency,this adjustment sometimes being termed trimming by rotating the shaft I4in its threaded mounting in the nut I5 until thc oscillator frequency ata predetermined point in its tuning range, as for example minimumfrequency position of the tuning knob, corresponds with a predeterminedCalibrating frequency, thereafter locking the shaft in correct positionby the lock nut 28. This having been done at room temperature, the unitwould then be put in a refrigerator or otherwise have its temperatureconsiderably reduced, say 50 degrees, and the adjusting bolts 24 and 2Swould then be moved in and out until the resonant frequency of thecircuit at this new temperature was exactly the same as that previouslyset at the other temperature. This is normally suilicient to provideaccurate temperature compensation throughout the entire range oftemperatures which may be encountered, although thisrlatter adjustmentmay be checked and corrected, if necessary, at still a third temperatureif desired.

The use of. a plate type of thermal or bi-metal strip, with both endsimmobilized, results in a very rugged and stable arrangement which doesnot introduce frequency flutter even under conditions of heavyvibration.

Another form of my invention is illustrated in Figure 5, the inductanceelement only of the oscillator being shown. Here the coil 50 hascooperating with its opposite ends two cores of such material that theyhave different permeability and provide, in combination with theirrcspectlve ends of the coil, different temperature coefilcients. Crossmembers 5I and 52 are rigidly fastened to opposite ends of the coil form53, the member 5I receiving and supporting a threaded shaft 54 ofinsulating material, this in turn carrying a `core 55; and the member 52receives a similar threaded shaft 55 carrying a core 51 of differentmaterial.

The ordinary powdered iron core, when used in combination with a coil,results in a positive temperature coefficient if the coil was designedto have a zero coeiiicient without the core, or increases the positivetemperature coefficient ii the coil already had such a coefllcient.Cores available on the open market, made with different metal particles,provide different temperature coefficients, at least one suchcommercially available core providing a negative temperaturecoefficient.

I take advantage of this resultant difference in temperaturecoeilicients by association of the coil with different core materials byhaving the cores 55 and 51 of material providing considerably diierenttemperature coefficients, one providing a temperature coeiiicient lyingto one side to the expected range of adjustment and the other to theother side. If a range about which adjustment was to be desired was inthe neighborhood of plus 30, one of these cores should be chosen ofmaterial providing a coefficient of about plus 50 with its end of thecoil, and the other in the neighborhood of plus 10, for example; whileif the desired range was in the neighborhood of zero, one core might bechosen to provide a positive temperature coefficient and the other anegative temperature coefficient. Since the cores' are independentlyadjustable, `bothwmay bem oved in or both out until the desiredinitialtrimming adjustment has lbeen made; then onemay be moved in and theother similarly moved out until the temperature compensating adjustmenthas been achieved. As before, this compensation can be merely for thecoil alone, but is preferably a compensation for theentire tank circuit.

The modicationof my invention illustrated in Figures 6 to 9 is anotherform employing a bimetal strip, the strip 60 in this case being formedin a spiral. The coil 6| again has a core 62 lying therewithin, thiscore being carried by a shaft 63 of insulating material. In this casethe core 62 is eccentricallymountedon the shaftv 63, in turn eccentricwith thel coil 6I. Rotational rather than longitudinal movement of thecore is used to vary its effect upon the coil, rotation of the shaftcentering the core or `throwing it considerably off centery as may bebest seen in Figure 9.

The shaft i-s rotatable in an appropriate bushing 64, and has movablymounted thereon a sleeve 65 provided with a slotted tapered outer endadapted to be wedged or jammed into gripping engagement with the shaft63 by the nut 66. I'he inner end of the spiral strip E0 is brazed orotherwise fastened lto the sleeve 65, and the outer end passes through aslot in a fixed bolt 61. .In making the initial adjustment the nut 66 isloosened and the shaft turned slightly one way or the other to effectthe trimming adjustment; then the nut 66 is tightened and more or lessof the spiral strip 6 pulled through the slot in the bolt 67 to effectadjustment of the compensation coefficient, since this latter actionvaries the effective length of the bi-metal strip. When the propercompensation coefficient has been attained the nut 68 is ltighteneddown.

While I have described certain embodiments of my invention, it is to beunderstood that it is capable of many modifications. Changes, therefore,in the construction and arrangement may be made Without departing fromthe spirit and scope of the invention as disclosed in the appendedclaims.

I claim:

1. In radio communications equipment, appa-- ratus of the characterdescribed, including: an impedance assembly having a movable portion; amember having at least a part movable as a function of temperaturevariations and connected to the movable portion of the impedanceassemblly for varying the impedance thereof; and means for adjusting theamount of movement of the movable part per unit of temperaturevariation.

2. Apparatus of the character claimed in claim 1, wherein the movablemember comprises a bimetal strip and the adjusting means comprises meansfor varying the effective length of the movable part of the strip.

3. In radio communications equipment, apparatus of the characterdescribed, including: an impedance element; a cooperating elementeffecting variation in the impedance of said first mentioned elementupon relative movement between Isaid elements; a bi-metal strip formoving one of said elements as a function of temperature variations, theconnection between the movable element and the strip being intermediatethe ends of the strip; and adjusting means for immobilizing a desiredportion of the strip, this means being movable toward andv away fromsaid connection. n U

` 4. Apparatus ofthe character claimed in claim 3, wherein at least aportion of the strip on each side of saidv connection is immobilized.

5. In radio communications equipment, apparatus of the characterdescribed, including: a tuned circuit having at least one inductiveelement and one capacitive element therein, one of said elements beingtunable `and the other element not being tunable in normal use; meansmovable as a function of temperature variations for varying thereactance of the other elements; and means for readily adjusting theamount of movement of the movable means per unit of .temperaturevariation to enable compensation such that the impedance of the entirecircuit `remains unchanged during temperature variations.

6.`In radio communications equipment, apparatus of the characterdescribed, including: a tuned circuit including a variable condenser anda coil; a core adapted to affect the inductance of said coil; and amember carrying the core; a bi-metal strip carrying said member andadapted to move the member and core as a function of temperaturevariations, the member being movably mounted on said strip to provide atrimming adjustment.

7. Apparatus of the character claimed in claim 6, including means forvarying the effective length of the strip to adjust the amount ofmovement of the core per unit of temperature variation. A

8. Apparatus of the character claimed in claim 1, wherein the movablemeans includes a spiral bi-netal strip and the adjusting means comprisesmeans for varying the effective length of the strip.

9. In radio communications equipment, apparatus of the characterdescribed, including: a tuned circuit including a variable condenser anda coil; a core adapted to aiect the inductance of said coil; a membercarrying the core; a spiral bi-metal strip adapted to move the memberand the core as a function of temperature variations; and means forvarying the effective length of the strip to adjust the amount ofmovement of the core per unit of temperature variation.

10. Apparatus of the character claimed in claim 9, wherein the core iseccentrically mounted within the coil and Ithe member and core arerotatable.

11. In radio communications equipment, apparatus of the characterdescribed, including: a tuned circuit including a variable condenser anda coil; a powdered metal core adapted to affect the inductance of saidcoil; a member carrying the core; and a bi-metal plate carrying saidmember and adapted to move the member and core as a function oftemperature variations, the member being movably mounted on said plateat the center thereof, the movement between the member and plate beingadapted to provide a trimming adjustment.

12. In radio communications equipment, apparatus of the characterdescribed, including an inductive element comprising a cylindrical coil;compensating means operatively associated with said element foraffecting the inductance of the element upon temperature variations,said compensating means comprising a rotatable core adapted to affectlthe inductance of the inductive element, this core being so mountedthat its axis of rotation is eccentric with respect to the axis of saidcoil; and means for adjusting the amount of rotation of said core perunit of temperature change effecting operation of said compensatingmeans.

THEODORE A. HUNTER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date Hopkins Nov. 11, 1913 Scott. Nov.30, 1937 Bell Sept. 26, 1939 Moore Dec. 9, 1941 Marrison Dec. 15, 1931Polydoroi Jan. 17, 1939 Harvey Nov. 21, 1939 Davis Oct. 25, 1932 Number15 Number

